1. Field of the Invention.
The present invention relates to sealing mechanisms and more particularly to seal rings adapted to serve as seat rings in a valve.
2. Description of the Prior Art.
In U.S. Pat. No. 3,642,248, issued to Robert E. Benware and owned by the assignee of the present invention, there is disclosed a valve sealing mechanism in the form of a shouldered wear ring confined in an undercut circumferential groove. The wear ring has a continuous central rib extending radially beyond the shouldered portion through the opening in the groove, the circumferential surface of the rib serving as a seating surface in mating contact with a complementary valve member sealing surface.
The wear ring in the Benware patent preferably is combined with an elastomeric O-type backing ring positioned between the bottom of the groove and the shouldered portion of the wear ring. The backing ring serves the dual function of urging the wear ring out of the groove into firm contact with the sealing surface and of sealing against leakage of fluid under pressure behind the wear ring.
An important application of butterfly valves of the general type described in U.S. Pat. No. 3,642,248 is in systems handling fluids at high pressure and temperature or flammable fluids that present a substantial risk of fire and resultant high temperatures. The patent recommends for such applications that the wear ring be fabricated of fully work hardened stainless steel and have a pair of outwardly flared side flanges extending one from each side of the shouldered portion in a generally radial direction opposite to the central rib (column 10, line 30 through column 11, line 32).
The flare of the side flanges is designed to provide a preloaded lateral sealing force against the sidewalls of the groove. The inner circumference of the wear ring seating surface is designed to provide a preloaded radial sealing force between the wear ring seating surface and the sealing surface of the valve member. By thus laterally and radially preloading the stainless steel wear ring, positive sealing is provided both at the seating surface-sealing surface interface and at the lines of contact between the side flanges and the walls of the groove, even if excessive temperatures should damage or destroy the elastomeric backing ring.
Although the stainless steel wear ring is an effective "fire-safe" seal, the hard metal-to-metal contact yields a leakage rate higher than that experienced with wear rings fabricated from various plastic materials.
A subsequent patent (U.S. Pat. No. 3,986,699 issued to Joseph A. Wucik, Jr., et al. and also assigned to the assignee of the present invention) describes a "fire-safe" seal having a soft nonmetallic seal ring of the Benware type arranged in an undercut groove of revised design. The downstream lip of the groove in the Wucik et al. U.S. Pat. No. 3,986,699 is in the form of an annular thin-walled skirt having an inner circumference adapted to make preloaded sealing contact with a mating sealing surface of the valve member.
Thus, the improved arrangement of the Wucik et al. '699 patent provided a soft seating ring, for superior sealing under ordinary temperature conditions. The soft seating ring was then backed up by a hard metal-to-metal seal between the skirt-like lip and the sealing surface of the valve member.
Although the Wucik et al. "fire-safe" seal design is effective for its intended purpose, it has several practical drawbacks. First of all, it can be used only for installations where pressure is exerted always from the same direction, since the backup skirt seal must be downstream of the soft nonmetallic seal ring.
Secondly, the sealing lip, as disclosed, forms one side of the groove in a seating region of a valve, such as a butterfly trunnion valve. The dimensional tolerances of the lip are much closer than are needed for the rest of the groove elements, and concentricity of the lip with the valve member is important. Consequently, more than usual care and expense is involved in machining and assembling valves having this type of dual seal.
Increasingly stringent environmental requirements, placed particularly upon petroleum drilling and processing facilities, have resulted in a need for valves having the capability to withstand very high temperatures that may result from a fire, for example, and still provide very high sealing effectiveness after such a fire. The dual type of soft seal and hard metal backup seal have not proved able to meet such stringent requirements when installed in a wafer trunnion valve. In fact, it has been considered in the industry that only ball valves would be capable of providing the required "fire-safe" operation.
For example, the Oil Companies Materials Association (OCMA) has issued test specifications (OCMA Specification No. FSV-1: Fire Safe Test For Soft Seat Ball Valves) for a ball valve having a soft seat (e.g., a nonmetallic plastic or soft metal seat) for a high degree of sealing effectiveness under normal conditions. After exposure to test temperatures that completely destroy the soft seat, the valve must subsequently have a leakage rate at full rated pressure of no more than 10 milliliters per minute per inch of valve diameter. In no case shall the leakage rate after destruction of the soft seal be greater than 100 milliliters per minute.
Another well-known oil industry specification of similar stringency is EXXON BP 3-14-1. Up to the present time no standard production-line ball valve has been able to meet either of these specifications, and wafer trunnion valves have performed even less well.